The IPM motor is a brushless motor having permanent magnets embedded in a rotor core. The IPM motor is advantageous in large output torque per volume, and reduced input voltage. These advantages make it preferable to apply the IPM motor to drive motors of electric vehicles.
An IPM motor also functions as a generator, when externally fed with motive energy. The fact that an IPM motor also functions as a generator is especially important for application to electric vehicles. Hereinafter, IPM motors and IPM generators are collectively referred to as IPM electric rotating machines, when it is unnecessary to distinguish IPM motors and IPM generators.
The IPM electric rotating machine is desired to generate a large output torque. An IPM motor structure for generating a large output torque is disclosed in Japanese Laid Open Patent Application No. Jp 2002-153033A. In the disclosed IPM motor, permanent magnets of the field magnet system are shallowly embedded in a rotor core. Such structure enables generating an increased magnet torque and an auxiliary reluctance torque, and thereby effectively increases the output torque of the IPM motor. The above-mentioned document discloses that the increase in the flux density of the flux applied to armature coils is preferable for increasing the magnet torque. Additionally, the above-mentioned document discloses that it is preferable that the permanent magnets are shallowly embedded in the rotor core, and thereby the q-axis inductance and the d-axis inductance satisfy the following relation:0≦(Lq−Ld)/Ld<0.3.